Low cross-talk multicore optical fiber for single mode operation

ABSTRACT

A multicore optical fiber comprises a common cladding and a plurality of core portions disposed in the common cladding. Each of the core portions includes a central axis, a core region extending from the central axis to a radius r1, the core region comprising a relative refractive index Δ1, an inner cladding region extending from the radius r1 to a radius r2, the inner cladding region comprising a relative refractive index Δ2, and a depressed cladding extending from the radius r2 to a radius r3, the depressed cladding region comprising a relative refractive index Δ3 and a minimum relative refractive index Δ3 min. The relative refractive indexes may satisfy Δ1&gt;Δ2&gt;Δ3 min. The mode field diameter of each core portion may greater than or equal to 8.2 μm and less than or equal to 9.5 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofU.S. Provisional Application Ser. No. 63/056,869 filed on Jul. 27, 2020,the content of which is relied upon and incorporated herein by referencein its entirety.

FIELD

The present disclosure pertains to optical fibers. More particularly,the present disclosure relates multicore optical fibers having coreportions with a relative refractive index profile including a trench forachieving low cross-talk.

TECHNICAL BACKGROUND

The transmission capacity through single-mode optical fiber hastheoretically reached its fundamental limit of around 100 Tb/s/fiber.Transmitting even 80 Tb/s in a single-mode optical fiber overtransoceanic distance has proven challenging in the absence of animproved optical signal-to-noise ratio (OSNR). The actual capacity limitover about 10,000 km of a submarine single-mode optical fiber systemused in a transoceanic transmission system is only about 50 Tb/s, evenwith advanced ultra-low-loss and low-nonlinearity optical fibers.

Multicore fiber (MCF) may exhibit less transmission loss when used intransoceanic applications. However, for practical use in ultra-long-haulsubmarine systems, the MCF should have ultra-low loss (i.e., lowattenuation) to produce a high OSNR, have high spatial-mode density toincrease the spatial channel count, and enable low differential groupdelay (DGD) between spatial modes to decrease the digital signalprocessing complexity. Also, the standard cladding diameter of 125 μmshould be maintained so that no major modifications are needed forinstallation of the cable.

SUMMARY

Thus, there is a need for new optical fibers that solve the problemsdescribed above while having satisfactory attenuation and an increasedtransmission capacity.

A first aspect of the present disclosure includes a multicore opticalfiber comprising a common cladding and a plurality of core portionsdisposed in the common cladding Each of the plurality of core portionscomprises a central axis, a core region extending from the central axisto a radius r1, the core region comprising a relative refractive indexΔ1 relative to pure silica, an inner cladding region encircling anddirectly contacting the core region and extending from the radius r₁ toa radius r₂, the inner cladding region comprising a relative refractiveindex Δ₂ relative to pure silica, and a depressed cladding regionencircling and directly contacting the inner cladding region andextending from the radius r₂ to a radius r₃, the depressed claddingregion comprising a relative refractive index Δ₃ relative to pure silicaand a minimum relative refractive index Δ_(3 min) relative to puresilica. In embodiments, the relative refractive indexes Δ₁, Δ₂, and Δ₃satisfy the relation Δ₁≥Δ₂>Δ_(3 min). The mode field diameter of eachcore portion is greater than or equal to 8.2 μm and less than or equalto 9.5 μm at a wavelength of 1310 nm. The zero dispersion wavelength ofeach core portion is greater than or equal to 1300 nm and less than orequal to 1324 nm.

A second aspect of the present disclosure may include the first aspect,wherein the common cladding comprises an outer radius R_(CC) that isgreater than or equal to 120 μm and less than or equal to 200 μm.

A third aspect of the present disclosure may include any of the firstthrough second aspects, wherein the plurality of core portions comprisesgreater than or equal to 3 core portions and less than or equal to 8core portions.

A fourth aspect of the present disclosure may include any of the firstthrough third aspects, wherein the outer radius R_(CC) is equal to 125μm.

A fifth aspect of the present disclosure may include any of the firstthrough fourth aspects, wherein the plurality of core portions arearranged in a 2×2 arrangement within the common cladding such that eachcentral axis of a core portion of the plurality of core portions isseparated from central axes of two adjacent core portions by a minimumcore-to-core separation distance greater than or equal to 35 μm.

A sixth aspect of the present disclosure may include any of the firstthrough fifth aspects, wherein a cable cutoff wavelength of each of theplurality of core portions is less than or equal to 1260 nm.

A seventh aspect of the present disclosure may include any of the firstthrough sixth aspects, wherein the relative refractive index Δ₃ of theentire depressed cladding region in each of the plurality of coreportions is less than or equal to Δ₂ such that the depressed claddingregion forms a depressed-index trench in a relative refractive indexprofile of each core portion.

An eighth aspect of the present disclosure may include any of the firstthrough seventh aspects, wherein the depressed-index trench in therelative refractive index profile of each core portion has a trenchvolume of greater than or equal to 30% Δ μm² and less than or equal to75% Δ μm².

A ninth aspect of the present disclosure may include any of the firstthrough eighth aspects, wherein the depressed-index trench in therelative refractive index profile of each core portion has a volume ofgreater than or equal to 40% Δ μm² and less than or equal to 70% Δ μm².

A tenth aspect of the present disclosure may include any of the firstthrough ninth aspects, wherein the depressed-index trench extends to theradius r₃, wherein r₃ is greater than or equal to 11 μm and less than orequal to 20 μm.

An eleventh aspect of the present disclosure may include any of thefirst through tenth aspects, wherein r₃ is greater than or equal to 12μm and less than or equal to 18 μm.

A twelfth aspect of the present disclosure may include any of the firstthrough eleventh aspects, wherein the minimum relative refractive indexΔ_(3 min) occurs at r₃.

A thirteenth aspect of the present disclosure may include any of thefirst through twelfth aspects, wherein the relative refractive index Δ₃of the depressed cladding region of each core portion monotonicallydecreases from Δ₂ at the radius r₂ to Δ_(3 min) at r₃.

A fourteenth aspect of the present disclosure may include any of thefirst through thirteenth aspects, wherein the relative refractive indexΔ₃ of the depressed cladding region of each core portion continuouslydecreases from Δ₂ at the radius r₂ to Δ_(3 min) at r₃ such that thetrench has a substantially triangular-shape.

A fifteenth aspect of the present disclosure may include any of thefirst through fourteenth aspects, wherein the relative refractive indexΔ_(3 min) of the depressed cladding region of each core portion is lessthan or equal −0.2% Δ and greater than or equal to −0.6% Δ.

A sixteenth aspect of the present disclosure may include any of thefirst through fifteenth aspects, wherein the depressed cladding regionof each core portion comprises a down-dopant having a concentration thatvaries with radial distance from the central axis such that thedepressed cladding region comprises a maximum down-dopant concentrationat r₃ and a minimum down-dopant concentration at r₂.

A seventeenth aspect of the present disclosure may include any of thefirst through sixteenth aspects, wherein the down-dopant is fluorine andthe maximum down-dopant concentration is greater than or equal to 1.2 wt% and less than or equal to 2.0 wt %.

An eighteenth aspect of the present disclosure may include any of thefirst through seventeenth aspects, wherein the maximum down-dopantconcentration is greater than or equal to 1.2 wt % and less than orequal to 1.8 wt %.

A nineteenth aspect of the present disclosure may include any of thefirst through eighteenth aspects, wherein the inner cladding region ofeach of the core portions is substantially free of the down-dopant.

A twentieth aspect of the present disclosure may include any of thefirst through nineteenth aspects, wherein the mode field diameter ofeach core portion at 1310 nm is greater than or equal to 8.8 μm and lessthan or equal to 9.5 μm.

A twenty first aspect of the present disclosure may include any of thefirst through twentieth aspects, wherein the mode field diameter of eachcore portion at 1310 nm is greater than or equal to 9.0 μm and less thanor equal to 9.5 μm.

A twenty second of the present disclosure may include any of the firstthrough twenty first aspects, wherein the mode field diameter of eachcore portion at 1310 nm is greater than or equal to 9.1 μm and less thanor equal to 9.5 μm.

A twenty third of the present disclosure may include any of the firstthrough twenty second aspects, wherein the central axes of the pluralityof core portions are separated from one another by a minimum separationdistance that is greater than or equal to 35 microns.

A twenty fourth aspect of the present disclosure may include any of thefirst through twenty third aspects, wherein a cross-talk between each ofthe plurality of core portions and a nearest one of the plurality ofcore portions is less than or equal to −30 dB.

A twenty fifth aspect of the present disclosure may include any of thefirst through twenty fourth aspects, wherein a cross-talk between eachof the plurality of core portions and a nearest one of the plurality ofcore portions is less than or equal to −50 dB.

A twenty sixth aspect of the present disclosure may include a multicoreoptical fiber comprising a common cladding and a plurality of coreportions disposed in the common cladding. each of the plurality of coreportions may comprise a central axis, a core region extending from thecentral axis to a radius r₁, the core region comprising a relativerefractive index Δ₁ relative to pure silica, an inner cladding regionencircling and directly contacting the core region and extending fromthe radius r₁ to a radius r₂, the inner cladding region comprising arelative refractive index Δ₂ relative to pure silica, and a depressedcladding region extending encircling and directly contacting the innercladding region and from the radius r₂ to a radius r₃, the depressedcladding region comprising a relative refractive index Δ₃ relative topure silica and a minimum relative refractive index Δ_(3 min) relativeto pure silica. The relative refractive indexes Δ₁, Δ₂, and Δ_(3 min)may satisfy the relations Δ₁>Δ₂>Δ_(3 min) and Δ₂≥Δ₃ such that thedepressed cladding region forms a depressed-index trench in a relativerefractive index profile of each core portion between r₂ and r₃. Inembodiments, Δ₃ monotonically decreases to the minimum relativerefractive index Δ_(3 min) with increasing radial distance from thecentral axis of each core portion.

A twenty seventh aspect of the present disclosure may include the twentysixth aspect, wherein the mode field diameter of each core portion at1310 nm is greater than or equal to 8.2 μm and less than or equal to9.5.

A twenty eighth aspect of the present disclosure may include any of thetwenty sixth through twenty seventh aspects, wherein the zero dispersionwavelength of each core portion is greater than or equal to 1300 nm andless than or equal to 1324 nm.

A twenty ninth aspect of the present disclosure may include any of thetwenty sixth through twenty eighth aspects, wherein the depressed-indextrench in the relative refractive index profile of each core portion hasa volume of greater than or equal to 40% Δ μm² and less than or equal to70% Δ μm².

A thirtieth aspect of the present disclosure may include any of thetwenty sixth through twenty ninth aspects, wherein a cable cutoffwavelength of each of the plurality of core portions is less than orequal to 1260 nm.

A thirty first aspect of the present disclosure may include any of thetwenty sixth through thirtieth aspects, wherein each core regioncomprises a maximum relative refractive index Δ_(1 max) relative to puresilica, wherein Δ_(1 max) in each of the core portions is greater thanor equal to 0.28% Δ and less than or equal to 0.45% Δ.

A thirty second aspect of the present disclosure may include any of thetwenty sixth through thirty first aspects, wherein the refractive indexprofile of each core portion within the core region is a graded indexprofile.

A thirty third aspect of the present disclosure may include any of thetwenty sixth through thirty second aspects, wherein an alpha value ofthe graded index profile is greater than or equal to 10.

A thirty fourth aspect of the present disclosure may include any of thetwenty sixth through thirty third aspects, wherein an alpha value of thegraded index profile is less than or equal to 5.

A thirty fifth aspect of the present disclosure may include any of thetwenty sixth through thirty fourth aspects, wherein r₃ is greater thanor equal to 12 μm and less than or equal to 18 μm.

A thirty sixth aspect of the present disclosure may include any of thetwenty sixth through thirty fifth aspects, wherein Δ₃ min is less thanor equal to −0.2% Δ and greater than or equal to −0.6% Δ.

A thirty seventh aspect of the present disclosure may include any of thetwenty sixth through thirty eighth aspects, wherein the common claddingcomprises an outer radius R_(CC) that is greater than or equal to 120 μmand less than or equal to 200 μm.

A thirty eighth aspect of the present disclosure may include any of thetwenty sixth through thirty ninth aspects, wherein the plurality of coreportions comprises greater than or equal to 3 core portions and lessthan or equal to 8 core portions.

A thirty ninth aspect of the present disclosure may include any of thetwenty sixth through thirty eighth aspects, wherein the relativerefractive index Δ₃ of the depressed cladding region of each coreportion continuously decreases from Δ₂ at the radius r₂ to Δ₃ min at r₃such that the trench has a substantially triangular-shape.

A fortieth aspect of the present disclosure may include a method offorming a multicore optical fiber. The method includes forming a coreregion of a core cane, the core region comprising an up-dopant; anddepositing an overclad layer around the core region to form a silicasoot pre-form; consolidating the silica soot pre-form in a consolidationfurnace. A time period T after beginning the consolidating of the silicasoot pre-form, the method includes exposing the silica soot pre-form toa down-dopant. The time period T is determined based on a rate ofdiffusion of the down-dopant trough the overclad layer such that aninner cladding region of the overclad layer is substantially free of thedown-dopant upon consolidation of the silica soot preform such that theconsolidated silica soot preform comprises the core region comprising arelative refractive index Δ₁ relative to pure silica, the inner claddingregion comprising a relative refractive index Δ₂ relative to puresilica, and a depressed cladding region comprising a relative refractiveindex Δ₃ relative to pure silica that decreases between Δ₂ and a minimumrelative refractive index Δ_(3 min). The method also includes insertingthe consolidated silica soot preform into a soot blank to form amulticore preform; and drawing the multicore fiber preform into themulticore optical fiber.

A forty first aspect of the present disclosure may include the fortiethaspect, wherein the overclad layer is deposited around the core regionusing an outside vapor deposition process.

A forty second aspect of the present disclosure may include any of thefortieth through the forty first aspects, wherein the up-dopantcomprises germanium.

A forty third aspect of the present disclosure may include any of thefortieth through the forty second aspects, wherein the down-dopantcomprises fluorine.

A forty fourth aspect of the present disclosure may include any of thefortieth through the forty third aspects, wherein the overclad layer isdeposited around the core region when the core region is in a partiallyconsolidated state such that the core region is consolidated inconjunction with the overclad layer.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary and are intendedto provide an overview or framework to understand the nature andcharacter of the claims. Additional features and advantages will be setforth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from the description orrecognized by practicing the embodiments as described in the writtendescription and claims hereof, as well as the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent disclosure, and together with the description serve to explainprinciples and operation of methods, products, and compositions embracedby the present disclosure, in which:

FIG. 1 schematically depicts an optical system including a signalsource, a multicore optical fiber, and a photodetector, according to oneor more embodiments shown and described herein;

FIG. 2 schematically depicts a cross-section of the multicore opticalfiber depicted in FIG. 1 , according to one or more embodimentsdescribed herein;

FIG. 3 schematically depicts a cross-section of a multicore opticalfiber, according to one or more embodiments described herein;

FIG. 4 schematically depicts a cross-section of a multicore opticalfiber, according to one or more embodiments described herein;

FIG. 5 schematically depicts a cross-section of a core portion of amulticore optical fiber comprising a core region, an inner claddingregion, and a depressed cladding region, according to one or moreembodiments described herein;

FIG. 6 graphically depicts a relative refractive index profile of a coreportion and common cladding, according to one or more embodimentsdescribed herein;

FIG. 7 graphically depicts a relative refractive index profile of a coreportion and common cladding, according to one or more embodimentsdescribed herein;

FIG. 8 depicts a flow diagram of a method of making a multimode opticalfiber comprising a core portion comprising a core region, an innercladding region, and a depressed cladding region, according to one ormore embodiments described herein; and

FIG. 9 schematically depicts a process of consolidating a core canewhile exposing the core cane to a down-dopant to create a depressedcladding region, according to one or more embodiments described herein.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of multicore opticalfibers examples of which are illustrated in the accompanying drawings.Whenever possible, the same reference numerals will be used throughoutthe drawings to refer to the same or like parts. One embodiment of amulticore optical fiber is shown in cross section in FIG. 1 . Themulticore optical fiber may include a plurality of core portions. Eachof the plurality of core portions may include a central axis and a coreregion extending from the central axis to a radius r₁. The core regioncomprising a relative refractive index Δ₁ relative to pure silica. Aninner cladding region may encircle and directly contact the core regionand extend from the radius r₁ to a radius r₂. The inner cladding regionmay have a relative refractive index Δ₂ relative to pure silica. Adepressed cladding region may encircle and directly contact the innercladding region and extending from the radius r₂ to a radius r₃. Thedepressed cladding region may include a relative refractive index Δ₃relative to pure silica and a minimum relative refractive index Δ₃ minrelative to pure silica. In this embodiment, Δ₁>Δ₂>Δ₃ min. The modefield diameter of each core portion at 1310 nm may be greater than orequal to 8.2 μm and less than or equal to 9.5 μm. The zero dispersionwavelength of each core portion is greater than or equal to 1300 nm andless than or equal to 1324 nm. Various embodiments of multicore opticalfibers will be described herein in further detail with specificreference to the appended drawings.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

As used herein, the term “about” means that amounts, sizes,formulations, parameters, and other quantities and characteristics arenot and need not be exact, but may be approximate and/or larger orsmaller, as desired, reflecting tolerances, conversion factors, roundingoff, measurement error and the like, and other factors known to those ofskill in the art. When the term “about” is used in describing a value oran end-point of a range, the disclosure should be understood to includethe specific value or end-point referred to. Whether or not a numericalvalue or end-point of a range in the specification recites “about,” thenumerical value or end-point of a range is intended to include twoembodiments: one modified by “about,” and one not modified by “about.”It will be further understood that the end-points of each of the rangesare significant both in relation to the other end-point, andindependently of the other end-point.

A multicore optical fiber, also referred to as a multicore optical fiberor “MCF”, is considered for the purposes of the present disclosure toinclude two or more core portions disposed within a common cladding.Each core portion can be considered as having a higher index core regionsurrounded by a lower index inner cladding region. As used herein, theterm “inner core portion” refers to the higher index core region. Thatis, a core portion may include an inner core portion and one or morelower index inner claddings.

“Radial position” and/or “radial distance,” when used in reference tothe radial coordinate “r” refers to radial position relative to thecenterline (r=0) of each individual core portion in a multicore opticalfiber. “Radial position” and/or “radial distance,” when used inreference to the radial coordinate “R” refers to radial positionrelative to the centerline (R=0, central fiber axis) of the multicoreoptical fiber.

The length dimension “micrometer” may be referred to herein as micron(or microns) or μm.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and radial distance r from the coreportion's centerline for each core portion of the multicore opticalfiber. For relative refractive index profiles depicted herein asrelatively sharp boundaries between various regions, normal variationsin processing conditions may result in step boundaries at the interfaceof adjacent regions that are not sharp. It is to be understood thatalthough boundaries of refractive index profiles may be depicted hereinas step changes in refractive index, the boundaries in practice may berounded or otherwise deviate from perfect step function characteristics.It is further understood that the value of the relative refractive indexmay vary with radial position within the core region and/or any of thecladding regions. When relative refractive index varies with radialposition in a particular region of the fiber (core region and/or any ofthe cladding regions), it may be expressed in terms of its actual orapproximate functional dependence or in terms of an average valueapplicable to the region. Unless otherwise specified, if the relativerefractive index of a region (core region and/or any of the inner and/orcommon cladding regions) is expressed as a single value, it isunderstood that the relative refractive index in the region is constant,or approximately constant, and corresponds to the single value or thatthe single value represents an average value of a non-constant relativerefractive index dependence with radial position in the region. Whetherby design or a consequence of normal manufacturing variability, thedependence of relative refractive index on radial position may besloped, curved, or otherwise non-constant.

The “relative refractive index” or “relative refractive index percent”as used herein with respect to multicore optical fibers and fiber coresof multicore optical fibers is defined according to equation (1):

$\begin{matrix}{{\Delta\%} = {100\frac{{n^{2}(r)} - n_{c}^{2}}{2{n^{2}(r)}}}} & (1)\end{matrix}$where n(r) is the refractive index at the radial distance r from thecore's centerline at a wavelength of 1550 nm, unless otherwisespecified, and n_(c) is 1.444, which is the refractive index of undopedsilica glass at a wavelength of 1550 nm. As used herein, the relativerefractive index is represented by Δ (or “delta”) or Δ % (or “delta %)and its values are given in units of “%” or “% Δ”, unless otherwisespecified. Relative refractive index may also be expressed as Δ(r) orΔ(r) %. When the refractive index of a region is less than the referenceindex n_(c), the relative refractive index is negative and can bereferred to as a trench. When the refractive index of a region isgreater than the reference index n_(c), the relative refractive index ispositive and the region can be said to be raised or to have a positiveindex.

The average relative refractive index of a region of the multicoreoptical fiber can be defined according to equation (2):

$\begin{matrix}{{\Delta\%} = \frac{\int_{r_{inner}}^{r_{outer}}{{\Delta(r)}dr}}{\left( {r_{outer} - r_{inner}} \right)}} & (2)\end{matrix}$where r_(inner) is the inner radius of the region, r_(outer) is theouter radius of the region, and Δ(r) is the relative refractive index ofthe region.

The term “α-profile” (also referred to as an “alpha profile”) refers toa relative refractive index profile Δ(r) that has the followingfunctional form (3):

$\begin{matrix}{{\Delta(r)} = {{\Delta\left( r_{0} \right)}\left\{ {1 - \left\lbrack \frac{❘{r - r_{0}}❘}{\left( {r_{1} - r_{0}} \right)} \right\rbrack^{\alpha}} \right\}}} & (3)\end{matrix}$where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) is zero, and r is in the range r_(i)≤r≤r_(f), where r_(i) isthe initial point of the α-profile, r_(f) is the final point of theα-profile, and α is a real number. In some embodiments, examples shownherein can have a core alpha of 1≤α≤100. In practice, an actual opticalfiber, even when the target profile is an alpha profile, some level ofdeviation from the ideal configuration can occur. Therefore, the alphaparameter for an optical fiber may be obtained from a best fit of themeasured index profile, as is known in the art.

The term “graded-index profile” refers to an α-profile, where α<10. Theterm “step-index profile” refers to an α-profile, where α≥10.

The “effective area” can be defined as (4):

$\begin{matrix}{A_{eff} = \frac{2{\pi\left\lbrack {\int_{0}^{\infty}{\left( {f(r)} \right)^{2}rdr}} \right\rbrack}^{2}}{\int_{0}^{\infty}{\left( {f(r)} \right)^{4}rdr}}} & (4)\end{matrix}$where f(r) is the transverse component of the electric field of theguided optical signal and r is radial position in the fiber. “Effectivearea” or “A_(eff)” depends on the wavelength of the optical signal.Specific indication of the wavelength will be made when referring to“Effective area” or “A_(eff)” herein. Effective area is expressed hereinin units of “μm²”, “square micrometers”, “square microns” or the like.

Unless otherwise noted herein, optical properties (such as dispersion,dispersion slope, etc.) are reported for the LP01 mode.

“Chromatic dispersion,” herein referred to as “dispersion” unlessotherwise noted, of an optical fiber is the sum of the materialdispersion, the waveguide dispersion, and the intermodal dispersion.“Material dispersion” refers to the manner in which the refractive indexof the material used for the optical core affects the velocity at whichdifferent optical wavelengths propagate within the core. “Waveguidedispersion” refers to dispersion caused by the different refractiveindices of the core and cladding of the optical fiber. In the case ofsingle mode waveguide fibers, the inter-modal dispersion is zero.Dispersion values in a two-mode regime assume intermodal dispersion iszero. The zero dispersion wavelength (λ₀) is the wavelength at which thedispersion has a value of zero. Dispersion slope is the rate of changeof dispersion with respect to wavelength. Dispersion and dispersionslope are reported herein at a wavelength of 1310 nm or 1550 nm, asnoted, and are expressed in units of ps/nm/km and ps/nm²/km,respectively. Chromatic dispersion is measured as specified by the IEC60793-1-42:2013 standard, “Optical fibres—Part 1-42: Measurement methodsand test procedures—Chromatic dispersion.”

The cutoff wavelength of an optical fiber is the minimum wavelength atwhich the optical fiber will support only one propagating mode. Forwavelengths below the cutoff wavelength, multimode transmission mayoccur and an additional source of dispersion may arise to limit thefiber's information carrying capacity. Cutoff wavelength will bereported herein as a cable cutoff wavelength. The cable cutoffwavelength is based on a 22-meter cabled fiber length as specified inTIA-455-80: FOTP-80 IEC-60793-1-44 Optical Fibres—Part 1-44: MeasurementMethods and Test Procedures—Cut-off Wavelength (21 May 2003), byTelecommunications Industry Association (TIA).

The bend resistance of an optical fiber, expressed as “bend loss”herein, can be gauged by induced attenuation under prescribed testconditions as specified by the IEC-60793-1-47:2017 standard, “Opticalfibres—Part 1-47: Measurement methods and test procedures—Macrobendingloss.” For example, the test condition can entail deploying or wrappingthe fiber one or more turns around a mandrel of a prescribed diameter,e.g., by wrapping 1 turn around either a 15 mm, 20 mm, or 30 mm orsimilar diameter mandrel (e.g. “1×15 mm diameter bend loss” or the “1×20mm diameter bend loss” or the “1×30 mm diameter bend loss”) andmeasuring the increase in attenuation per turn.

The term “attenuation,” as used herein, is the loss of optical power asthe signal travels along the optical fiber. Attenuation is measured asspecified by the IEC 60793-1-40:2019 standard entitled “Opticalfibres—Part 1-40: Attenuation measurement methods.”

As used herein, the multicore optical fiber can include a plurality ofcore portions, wherein each core portion can be defined as an i^(th)core portion (i.e., 1^(st), 2^(nd), 3^(rd), 4^(th), etc. . . . ). Eachi^(th) core portion can have an outer radius r_(Ci). In embodiments, theouter radius r_(Ci) of each core portion corresponds to an outer radiusr₃ of a depressed cladding region of that core portion. Each i^(th) coreportion is disposed within a cladding matrix of the multicore opticalfiber, which defines a common cladding of the multicore optical fiber.The common cladding includes a relative refractive index Δ_(CC) and anouter radius R_(CC).

According to one aspect of the present disclosure, the core region formsthe central portion of each core portion within the multicore opticalfiber and is substantially cylindrical in shape. When two regions aredirectly adjacent to each other, the outer radius of the inner of thetwo regions coincides with the inner radius of the outer of the tworegions. For example, in embodiments in which an inner cladding regionsurrounds and is directly adjacent to a core region, the outer radius ofthe core region coincides with the inner radius of the inner claddingregion.

An “up-dopant” is a substance added to the glass of the component beingstudied that has a propensity to raise the refractive index relative topure undoped silica. A “down-dopant” is a substance added to the glassof the component being studied that has a propensity to lower therefractive index relative to pure undoped silica. Examples of up-dopantsinclude GeO₂ (germania), Al₂O₃, P₂O₅, TiO₂, Cl, Br, and alkali metaloxides, such as K₂O, Na₂O, Li₂O, Cs₂O, Rb₂O, and mixtures thereof.Examples of down-dopants include fluorine and boron.

The term “crosstalk” in a multi-core optical fiber is a measure of howmuch power leaks from one core portion to another, adjacent coreportion. As used herein, the term “adjacent core portion” refers to thecore that is nearest to the reference core portion. In embodiments, allcore portions may be equally spaced from one another, meaning that allcore portions are adjacent one another. In other embodiments, the coreportions may not be equally spaced from one another, meaning that somecore portions will be spaced further from the reference core portionthan adjacent core portions are spaced from the reference core portion.The crosstalk can be determined based on the coupling coefficient, whichdepends on the refractive index profile design of the core portion, thedistance between the two adjacent core portions, the structure of thecladding surrounding the two adjacent core portions, and Δβ, whichdepends on a difference in propagation constant β values between the twoadjacent core portions (e.g., as described herein, two core portionshaving centerlines separated by a minimum core-to-core separationdistance). For two adjacent core portions with power P₁ launched intothe first core portion, then the power P₂ coupled from the first coreportion to the second core portion can be determined from coupled modetheory using the following equation (5):

$\begin{matrix}{P_{2} = {\frac{L}{L_{c}}\left\langle {\left( \frac{\kappa}{g} \right)^{2}{\sin^{2}\left( {g\Delta L} \right)}} \right\rangle P_{1}}} & (5)\end{matrix}$where < > denotes the average, Lis fiber length, κ is the couplingcoefficient between the electric fields of the two cores, ΔL is thelength of the fiber, L_(c) is the correlation length, and g is given bythe following equation (6):

$\begin{matrix}{g^{2} = {\kappa^{2} + \left( \frac{\Delta\beta}{2} \right)^{2}}} & (6)\end{matrix}$where Δβ is the mismatch in propagation constants between the LP01 modesin the two adjacent core portions when they are isolated. The crosstalk(in dB) is then determined using the following equation (7):

$\begin{matrix}{X = {{10{\log\left( \frac{P_{2}}{P_{1}} \right)}} = {10{\log\left( {\frac{L}{L_{c}}\left\langle {\left( \frac{\kappa}{g} \right)^{2}{\sin^{2}\left( {g\Delta L} \right)}} \right\rangle} \right)}}}} & (7)\end{matrix}$

The crosstalk between the two adjacent core portions increases linearlywith fiber length in the linear scale (equation (5)) but does notincrease linearly with fiber length in the dB scale (equation (7)). Asused herein, crosstalk performance is referenced to a 100 km length L ofoptical fiber. However, crosstalk performance can also be representedwith respect to alternative optical fiber lengths, with appropriatescaling. For optical fiber lengths other than 100 km, the crosstalkbetween cores can be determined using the following equation (8):

$\begin{matrix}{{X(L)} = {{X(100)} + {10\log\left( \frac{L}{100} \right)}}} & (8)\end{matrix}$For example, for a 10 km length of optical fiber, the crosstalk can bedetermined by adding “−10 dB” to the crosstalk value for a 100 km lengthoptical fiber. For a 1 km length of optical fiber, the crosstalk can bedetermined by adding “−20 dB” to the crosstalk value for a 100 km lengthof optical fiber. For long-haul transmission in an uncoupled-coremulticore fiber, the crosstalk should be less than or equal to −30 dB,less than or equal to −40 dB, or even less than or equal to −50 dB.

Techniques for determining crosstalk between cores in a multicoreoptical fiber can be found in M. Li, et al., “Coupled Mode Analysis ofCrosstalk in Multicore fiber with Random Perturbations,” in OpticalFiber Communication Conference, OSA Technical Digest (online), OpticalSociety of America, 2015, paper W2A.35, and Shoichiro Matsuo, et al.,“Crosstalk behavior of cores in multi-core portion under bentcondition,” IEICE Electronics Express, Vol. 8, No. 6, p. 385-390,published Mar. 25, 2011 and Lukasz Szostkiewicz, et al., “Cross talkanalysis in multicore optical fibers by supermode theory,” OpticsLetters, Vol. 41, No. 16, p. 3759-3762, published Aug. 15, 2016, thecontents of which are all incorporated herein by reference in theirentirety.

The phrase “coupling coefficient” κ, as used herein, is related to theoverlap of electric fields when the two cores are close to each other.The square of the coupling coefficient, κ², is related to the averagepower in core m as influenced by the power in other cores in themulticore optical fiber. The “coupling coefficient” can be estimatedusing the coupled power theory, with the methods disclosed in M.Koshiba, K. Saitoh, K. Takenaga, and S. Matsuo, “Analytical Expressionof Average Power-Coupling Coefficients for Estimating IntercoreCrosstalk in Multicore fibers,” IEEE Photonics J., 4(5), 1987-95 (2012);and T. Hayashi, T. Sasaki, E. Sasaoka, K. Saitoh, and M. Koshiba,“Physical Interpretation of Intercore Crosstalk in Multicore fiber:Effects of Macrobend, Structure Fluctuation, and Microbend,” OpticsExpress, 21(5), 5401-12 (2013), the contents of which are incorporatedby reference herein in their entirety.

The mode field diameter (MFD) is measured using the Petermann II methodand was determined from:MFD=2w  (9)

$\begin{matrix}{w = \frac{\int_{0}^{\infty}\left( {f(r)} \right)^{2}}{\int_{0}^{\infty}{\left( \frac{d{f(r)}}{dr} \right)^{2}rdr}}} & (10)\end{matrix}$where f(r) is the transverse component of the electric fielddistribution of the guided light and r is the radial position in thefiber. Unless otherwise specified, “mode field diameter” or “MFD” refersto the mode field diameter at 1310 nm.

Directional terms as used herein—for example up, down, right, left,front, back, top, bottom—are made only with reference to the figures asdrawn and are not intended to imply absolute orientation.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order, nor that with any apparatus specificorientations be required. Accordingly, where a method claim does notactually recite an order to be followed by its steps, or that anyapparatus claim does not actually recite an order or orientation toindividual components, or it is not otherwise specifically stated in theclaims or description that the steps are to be limited to a specificorder, or that a specific order or orientation to components of anapparatus is not recited, it is in no way intended that an order ororientation be inferred, in any respect. This holds for any possiblenon-express basis for interpretation, including: matters of logic withrespect to arrangement of steps, operational flow, order of components,or orientation of components; plain meaning derived from grammaticalorganization or punctuation, and; the number or type of embodimentsdescribed in the specification.

As used herein, the term “substantially free,” when used to describe theconcentration and/or absence of a particular up-dopant or down-dopant ina particular portion of the fiber, means that the constituent componentis not intentionally added to the fiber. However, the fiber may containtraces of the constituent component as a contaminant or tramp in amountsof less than 0.15 wt. %.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a” component includes aspects having two or moresuch components, unless the context clearly indicates otherwise.

Multicore optical fibers are attractive for a number of optical fiberapplications, including their use for increasing fiber density toovercome cable size limitations and duct congestion issues in passiveoptical network (“PON”) systems. For example, multicore optical fibersare being considered for data center applications and high speed opticalinterconnects. In such applications, it is beneficial to increase thefiber density to maintain compactness of the multicore optical fiber(e.g., to provide a multicore optical fiber having a diameter to matchthe diameter of conventional optical fiber used for such applications,such as diameters of approximately 125 μm) while providing relativelyhigh fiber counts as compared to conventional optical fibers used forsuch applications. Conventional approaches for achieving high fiberdensities within such multimode optical fibers while reducing cross-talkbetween the cores of the multicore optical fiber have included reducingthe mode field diameter of each core portion at 1310 nm to less than 8.0μm. Such reduction in mode field diameter may reduce cross-talk betweenthe core portions, but creates difficulties in coupling each coreportion to a standard single mode fiber in optical interconnects,leading to signal losses.

The multimode optical fibers described herein address these deficienciesof conventional approaches to providing high fiber density in relativelysmall multicore optical fibers. In particular, by incorporating coreportions comprising a core region, an inner cladding region, and adepressed cladding region with a trenched refractive index profile, themultimode optical fibers described herein provide relatively lowcross-talk (e.g., less than −30 dB, less than −40 dB, or even less than−50 dB), low tunneling loss from corner fibers to the edge, and goodbending performance. Additionally, the multimode optical fibersdescribed herein achieve relatively large mode field diameters at 1310nm (e.g., greater than or equal to 8.2 μm and less than or equal to 9.5μm) for improved coupling to standard single mode fiber overconventional approaches.

Referring now to FIG. 1 , an optical system 100 comprising anuncoupled-core multicore optical fiber 110 with a plurality of coreportions C₁, C₂, C₃, and C₄ (FIG. 2 ), a signal source 180, and aphotodetector 190 is schematically depicted. The signal source 180 mayproduce multiple modulated signals such as those produced by distributedfeedback lasers (DFB) or vertical-cavity surface-emitting lasers(VCSEL). The uncoupled-core multicore optical fiber 110 comprises aninput end 112 optically coupled to the signal source 180, an output end114 optically coupled to the photodetector 190, and an outer surface116. In operation, the signal source 180 may selectively direct photonsfrom one laser into an individual core portion of the plurality of coreportions C₁, C₂, C₃, and C₄. For example, the signal source 180, theinput end 112 of the uncoupled-core multicore optical fiber 110, orboth, may be coupled to a multicore fan-in device, which is configuredto align the signal source 180 with any individual core portion of theplurality of core portions C₁, C₂, C₃, and C₄ (see FIG. 2 ).

FIG. 2 depicts a cross-sectional view of the uncoupled-core multicoreoptical fiber 110 along section II-II of FIG. 1 . The uncoupled-coremulticore optical fiber 110 includes a central fiber axis 12 (thecenterline of the uncoupled-core multicore optical fiber 110, whichdefines radial position R=0) and a common cladding 19. The commoncladding 19 can have an outer radius R_(CC), which in the depictedembodiment of FIG. 2 corresponds to the outer radius of theuncoupled-core multicore optical fiber 110. A plurality of core portionsC_(i) (individually denoted C₁, C₂, C₃, and C₄ in the example of FIG. 2and collectively referred to as core portions “C”) are disposed withinthe common cladding 19, with each core portion C_(i) generally extendingthrough a length of the uncoupled-core multicore optical fiber 110parallel to the central fiber axis 12.

In embodiments, 2*R_(CC) (e.g., the diameter of the multicore opticalfiber 110) is equal to 125 microns. In embodiments, the diameter of themulticore optical fiber 110 is greater than 140 microns. In embodiments,the diameter of the multicore optical fiber 110 is greater than 170microns. In embodiments, the diameter of the multicore optical fiber 110is less than 200 microns. In embodiments, the diameter of the classfiber is less than 160 microns. In embodiments, the diameter of themulticore optical fiber 110 is greater than or equal to 120 and lessthan or equal to 130 microns.

Each core portion C₁, C₂, C₃, and C₄ includes a central axis orcenterline CL₁, CL₂, CL₃ and CL₄ (which define radial position r=0 foreach core portion) and an outer radius r_(C1), r_(C2), r_(C3) andr_(C4). A position of each of the centerlines CL₁, CL₂, CL₃ and CL₄within the uncoupled-core multicore optical fiber 110 can be definedusing Cartesian coordinates with the central fiber axis 12 defining theorigin (0,0) of an x-y coordinate system coincident with the coordinatesystem defined by the radial coordinate R. The position of centerlineCL₁ can be defined as (x₁,y₁), the position of centerline CL₂ can bedefined as (x₂,y₂), the position of centerline CL₃ can be defined as(x₃,y₃), and the position of centerline CL₄ can be defined as (x₄,y₄).In embodiments, each of the core portions C_(i) is separated from anearest one (e.g., the core portion C_(i) having a center line CL_(i)that is closest to the centerline of that core portion) by a minimumcore-to-core separation distance (or “minimum separation distance”). Inembodiments, each of the core portions C_(i) is separated from multiplecore portions by the minimum separation distance. For example, asdepicted in FIG. 2 , the core portions C₁, C₂, C₃, and C₄ are arrangedin a 2×2 arrangement with each of centerlines CL₁, CL₂, CL₃ and CL₄being at the corner of a square. In such an arrangement, the centerlinesCL₁ and CL₂ of the core portions C₁ and C₂ are separated by a minimumseparation distance that can be defined asD_(c1)−D_(c2)=√[(x₂−x₁)²+(y₂−y₁)²]. The centerlines CL₁ and CL₄ of thecore portions C₁ and C₄ are also separated by the minimum separationdistance that can then be defined as D_(c1)−D_(c4)=√[x₄−x₁)²+(y₄−y₁)²].As used herein, the term “adjacent core portion” is used to denote coreportions having centerlines that are most proximate to one another(i.e., there is no other core portion C_(i) having a centerline CL_(i)that is more proximate to a core portion than an adjacent core portion).In embodiments, centerlines of adjacent core portions are separated bythe minimum separation distance. It should be understood that aparticular core portion may have multiple adjacent core portions.

In embodiments, the minimum separation distance between the coreportions C₁, C₂, C₃, and C₄ is greater than or equal to 35 microns tofacilitate maintaining a relatively low cross-talk between the coreportions C₁, C₂, C₃, and C₄. In embodiments, the minimum separationdistance is greater than or equal to 40 microns. In embodiments, theminimum separation distance is greater than or equal to 45 microns (e.g.greater than or equal to 50 microns, greater than or equal to 60microns, greater than or equal to 70 microns, greater than or equal to75 microns).

In embodiments, edges of the plurality of core portions C_(i) may alsobe spaced apart from the outer surface 116 of the uncoupled-coremulticore optical fiber 110 by at least a minimum core edge to fiberedge distance D_(E) as measured from the edge of each of the pluralityof core portions C_(i) to the outer surface 116. As depicted in FIG. 2 ,the minimum core edge to fiber edge distance D_(E) is the minimumdistance from a point along the outer circumference (e.g., a point onthe outer circumference that is closest to the outer surface 116) of acore portion C_(i) (e.g., corresponding to the r₃ value for the coreportion C₁, as described herein with respect to FIG. 5 ) to a nearestpoint along the circumference of the outer surface 116, as determined bya line segment between the point along the outer circumference of thecore portion C_(i) and the nearest point along the circumference on theouter surface 15 in a plan perpendicular to the fiber axis 12. Inembodiments, D_(E) is greater than or equal 8 microns. In embodiments,D_(E) is greater than or equal 12 microns. In embodiments, D_(E) isgreater than 15 microns. Without intending to be bound by any particulartheory, it is believed that the extent of signal loss due to tunnelingis dependent upon the minimum value for D_(E).

In embodiments, the uncoupled-core multicore optical fiber 110 can havea circular cross-section shape. It should be understood that themulticore optical fiber 110 may comprise different numbers of coreportions than those described with respect to FIGS. 1-2 and thearrangement of the core portions within the common cladding 19 (see FIG.1 ) may vary. In embodiments, the uncoupled-core multicore optical fiber110 can have N number of total core portions C_(i), wherein i=1 . . . Nand N is at least 3. According to one aspect of the present disclosure,the total number N of cores C_(i) in the uncoupled-core multicoreoptical fiber 110 is from 3 to 12, 3 to 10, 3 to 8, 3 to 6, 3 to 5, or 3to 4. For example, the total number N of core portions C_(i) in theuncoupled-core multicore optical fiber 10 can be 3, 4, 5, 6, 7, 8, 9,10, 11, 12, or any total number N of core portions C_(i) between any ofthese values. The total number N of core portions C_(i) can be even orodd and can be arranged in any pattern within the common cladding 19,non-limiting examples of which include a 2×2 pattern (or multiplesthereof, such as a 2×4 pattern), a rectangular pattern, a squarepattern, a rectangular pattern, a circular pattern, and a hexagonallattice pattern.

For example, FIG. 3 depicts a cross-sectional view of an uncoupled-coremulticore optical fiber 400 having N=7 core portions C_(i) arranged in ahexagonal lattice pattern. In embodiments, the uncoupled-core multicoreoptical fiber 400 may be used in place of the uncoupled-core multicoreoptical fiber 110 described with respect to FIG. 1 . The uncoupled-coremulticore optical fiber 400 includes a first core portion C₁ extendingthrough a central axis 412 of the uncoupled-core multicore optical fiber400. Six additional core portions C₂, C₃, C₄, C₅, C₆, and C₇ aredisposed in a cladding matrix 410 equidistantly from the first coreportion C₁ in a hexagonal arrangement. In embodiments, triplets of thecore portions including the first core portion C₁ and two of theadditional core portions C₂, C₃, C₄, C₅, C₆, and C₇ form equilateraltriangles with the centerlines of the core portions in each tripletseparated by a minimum separation distance equal to D_(C1)-D_(C2). Inembodiments, the minimum separation distance D_(C1)-D_(C2) is greaterthan or equal to 35 microns to facilitate maintaining relatively lowcross-talk between the core portions C₁, C₂, C₃, and C₄. In embodiments,the minimum separation distance is greater than or equal 40 microns. Inembodiments, the minimum separation distance D_(C1)-D_(C2) is greaterthan or equal to 45 microns. In embodiments, the arrangement of coreportions is centered within the cladding matrix 410 such that each ofthe additional core portions C₂, C₃, C₄, C₅, C₆, and C₇ is separatedfrom an outer surface 420 of the cladding matrix 410 by at least aminimum core edge to fiber edge distance D_(E). In embodiments, D_(E) isgreater than or equal 8 microns. In embodiments, D_(E) is greater thanor equal 12 microns. In embodiments, D_(E) is greater than 15 microns.

FIG. 4 depicts a cross-sectional view of an uncoupled-core multicoreoptical fiber 500 having N=3 core portions C_(i) arranged in atriangular pattern around a central axis 512. In embodiments, theuncoupled-core multicore optical fiber 500 may be used in place of theuncoupled-core multicore optical fiber 110 described with respect toFIG. 1 . The uncoupled-core multicore optical fiber 500 includes coreportions C₁, C₂, C₃ disposed in a cladding matrix 510. In embodiments,the core portions C₁, C₂, C₃ form an equilateral triangle with thecenterlines of the core portions in each triplet separated by aseparation distance equal to D_(C1)-D_(C2). In embodiments, the coreportions C₁, C₂, and C₃ are not evenly spaced (e.g., a centerline of thefirst core portion C_(i) may be separated from the second core portionC₂ by a first distance, and the second core portion C₂ may be separatedfrom the third core portion C₃ by a second difference that is differentfrom the first distance). In embodiments, the D_(C1)-D_(C2) is greaterthan the minimum separation distance described herein. To facilitatemaintaining relatively low cross-talk between the core portions C₁, C₂,and C₃. In embodiments, the minimum separation distance is greater thanor equal 35 microns or greater than or equal to 40 microns. Inembodiments, the minimum separation distance D_(C1)-D_(C2) is greaterthan or equal to 45 microns. In embodiments, each of the core portionscore portions C₁, C₂, C₃ is separated from an outer surface 520 of thecladding matrix 410 by at least a minimum core edge to fiber edgedistance D_(E). In embodiments, D_(E) is greater than or equal 8microns. In embodiments, D_(E) is greater than or equal 12 microns. Inembodiments, D_(E) is greater than 15 microns.

It should be appreciated that various numbers and arrangements of coreportions for the uncoupled-core multicore optical fiber 110 arecontemplated and possible. For example, in embodiments, theuncoupled-core multicore optical fiber 110 can have N=12 core portionsC_(i) arranged in a circular pattern. In embodiments, the uncoupled-coremulticore optical fiber 110 can have a core portion C_(i) positionedsuch that the core centerline CL_(i) aligns with the central fiber axis12. In embodiments, the uncoupled-core multicore optical fiber 110 canhave a core portion C_(i) pattern such that the cores C_(i) are spacedaround the central fiber axis 12.

FIG. 5 schematically depicts a cross sectional view of one of the coreportions C_(i) described herein with respect to FIGS. 1-2 along the lineV-V of FIG. 2 . In embodiments, each of the core portions C_(i) comprisea core region 150 centered on a centerline CL_(i) and a cladding region155. The cladding region 155 comprises an inner cladding region 160(also referred to herein as an inner cladding layer) encircling anddirectly contacting the core region 150 and a depressed cladding region170 encircling and directly contacting the inner cladding region 160. Inembodiments, the core region 150 and the cladding region 155 areconcentric such that the cross section of the core portion C_(i) isgenerally circular symmetric with respect to the centerline CL_(i)having an overall radius r_(Ci). The core region 150 has a radius r₁ andthe depressed cladding region 170 has a radius r₃ that defines an outerradius of the core portion C_(i) such that r₃ corresponds to the radiusr_(Ci) associated with each core portion C_(i) described herein withrespect to FIG. 2 . The inner cladding region 160 extends between theradius r₁ of the core region 150 and an inner radius r₂ of the depressedcladding region 170 such that the inner cladding region 160 has athickness T₂=r₂−r₁ in the radial direction. The depressed claddingregion 170 has a thickness T₃=r₃−r₂ in the radial direction. Thestructure, compositions, and optical properties of each of the coreregion 150, the inner cladding region 160, and the depressed claddingregion 170 are described in greater detail herein.

Referring to FIGS. 5 and 6 , a radial cross section of one embodiment ofone of the core portions C_(i) (FIG. 5 ) and corresponding relativerefractive index profile (FIG. 6 ) of the core portion C_(i) along theline VI in FIG. 2 are schematically depicted. In FIG. 6 , the relativerefractive index profile of the core portion C_(i) is plotted as afunction of radial distance r from the centerline CL_(i) of the coreportion C_(i). As depicted in FIG. 2 , the relative refractive indexprofile depicted in FIG. 6 extends radially outward from a centerlineCL_(i) of the core portion Ci and into a portion of the common cladding19. As depicted in FIG. 6 , the core region 150 has a relativerefractive index Δ_(L) In embodiments, the relative refractive index Δ₁may vary with radial coordinate (radius) r and be represented as Δ₁(r).In embodiments, the core region 150 comprises silica-based glass havingan up-dopant (e.g., germanium). In embodiments, the relative refractiveindex Δ₁(r) includes a maximum relative refractive index Δ_(1 max)(relative to pure silica). In embodiments, Δ_(1 max) is greater than orequal 0.28% Δ and less than or equal to 0.45% Δ. In embodiments, toachieve these values for Δ_(1 max), the core region 150 possesses anup-dopant (e.g., germanium) concentration of greater than or equal to 6wt. % and less than or equal to 9 wt. %. The up-dopant concentration mayvary within the core region 150. Providing a core portion C_(i) with aΔ_(1 max) value within this range facilitates each core portion C_(i)having a mode field diameter at 1310 nm greater than or equal to 8.2 μmand less than or equal to 9.5 μm.

In embodiments, the relative refractive index Δ₁(r) follows a gradedindex profile, with an α value of greater than or equal to 1.5 and lessthan or equal to 5.0. For example, in embodiments, the maximum relativerefractive index Δ_(1 max) may occur at r=0 (e.g., at the centerlineCL_(i)) and decrease with an alpha profile until reaching a radius r₁.In embodiments, the relative refractive index Δ₁(r) follows a step indexprofile, with an α value of greater than or equal to 10. For example, inembodiments, relative refractive index Δ₁(r) may remain substantiallyequal to the maximum relative refractive index Δ_(1 max) until theradius r₁. In embodiments, the radius r₁ coincides with an inner radiusof inner cladding region 160. In embodiments, the core radius r₁ isgreater than or equal to 3.0 microns and less than or equal to 7.0microns. In embodiments, the core radius r₁ is greater than or equal to3.5 microns and less than or equal to 6.5 microns (e.g., greater than orequal to 4.0 microns and less than or equal to 6.0 microns). Providing acore radius r₁ within this range facilitates each core portion C₁ havinga mode field diameter at 1310 nm greater than or equal to 8.2 μm andless than or equal to 9.5 μm.

Referring still to FIGS. 5 and 6 , the inner cladding region 160 extendsfrom radius r₁ to a radius r₂ such that the inner cladding has a radialthickness T₂=r₂−r₁. In embodiments, the inner cladding region 160comprises a relative refractive index Δ₂. In embodiments, the innercladding region 160 is formed from silica-based glass that issubstantially free of dopants (e.g., up-dopants and down-dopants) suchthat the relative refractive index Δ₂ is approximately 0. Inembodiments, the inner cladding region 160 is formed from a similarsilica-based glass as the common cladding 19 such that 42=Δ_(CC).Without wishing to be bound by theory, it is believed that the value ofr₂ (and hence the radial thickness T₂ of the inner cladding region 160)in part determines the zero dispersion wavelength of each of the coreportions C_(i). In embodiments, each of the core portions C_(i) has azero dispersion wavelength of greater than or equal 1300 nm and lessthan or equal to 1324 nm. To achieve such a zero dispersion wavelength,r₂ may be greater than or equal to 4.5 μm and less than or equal to 17μm. In embodiments, r₂ is greater than or equal 7.0 μm and less than orequal 7.5 μm.

The depressed cladding region 170 extends from the radius r₂ to theradius r₃ such that the outer cladding has a radial thickness T₃=r₃−r₂.The radius r₃ may correspond to the outer radius r_(Ci) of the radius ofa core portion C_(i) described herein with respect to FIG. 2 . Inembodiments, each core portion C_(i) has an outer diameter d=2*r₃.Without wishing to be bound by theory, it is believed that the value ofr₃ (and hence the radial thickness T₃ of the depressed cladding region170) in part determines a zero dispersion wavelength of each of the coreportions C_(i). In embodiments, each of the core portions C_(i) has azero dispersion wavelength of greater than or equal 1300 nm and lessthan or equal to 1324 nm, as noted herein. To achieve such a zerodispersion wavelength, r₃ may be greater than or equal to 11 μm and lessthan or equal to 20 μm. In embodiments, r₃ may be greater than or equalto 12 μm and less than or equal to 18 μm. In embodiments, r₃ may begreater than or equal to 14.5 μm and less than or equal to 16 μm.

The depressed cladding region 170 has a relative refractive index Δ₃. Inembodiments, the relative refractive index Δ₃ is less than or equal tothe relative refractive index Δ₂ of the inner cladding region 160throughout the depressed cladding region 170. The relative refractiveindex Δ₃ may also be less than or equal to the relative refractive indexΔ_(CC) of the common cladding 19 (see FIG. 2 ) such that the depressedcladding region 170 forms a trench in the relative refractive indexprofile of the core portion C_(i). The term “trench,” as used herein,refers to a region of the core portion that is, in radial cross section,surrounded by regions of the multicore fiber (e.g., the inner claddingregion 160 and the common cladding 19) having relatively higherrefractive indexes. In embodiments, the relative refractive index Δ₃ maybe constant throughout the depressed cladding region 170. In otherembodiments, the relative refractive index Δ₃ may vary with radialcoordinate r (radius) and be represented as Δ₃(r). In embodiments, therelative refractive index Δ₃(r) within the depressed cladding region 170decreases monotonically with increasing radial distance from thecenterline CL_(i) such that the depressed cladding region 170 comprisesa minimum relative refractive index 43 min at the r₃. In embodiments,Δ₃(r) decreases at a constant rate with radial distance from thecenterline CL_(i) such that the relative refractive index profile withinthe depressed cladding region 170 is substantially linear. Inembodiments, the relative refractive index Δ₃(r) continuously decreaseswith increasing radial distance from the centerline CL_(i) at anincreasing or decreasing rate such that the relative refractive indexprofile within the depressed cladding region 170 has a parabolic orsimilar shape that is either concave or convex. Referring still to FIGS.5-6 , in embodiments Δ₁>Δ₂>Δ₃ mm. In embodiments, Δ₂≥Δ₃ such that thedepressed cladding region forms a depressed-index trench in a relativerefractive index profile of each core portion between r₂ and r₃

Referring still to FIGS. 5 and 6 , in embodiments, the depressedcladding region 170 comprises silica glass having one or moredown-dopants (e.g., fluorine). In embodiments, the down-dopantconcentration within the depressed cladding region 170 varies as afunction of radial distance from the centerline CL₁ of the core portionC_(i). For example, in embodiments, the down-dopant concentration varieswithin the depressed cladding region 170 by increasing monotonicallyfrom, for example, a minimum value of 0 wt. % at the radial position r₂to a maximum value at the radial position r₃. In embodiments, themaximum value of the down-dopant concentration is greater than or equalto 1.2 wt. % and less than or equal to 2.0 wt. %. In embodiments, themaximum fluorine concentration F_(max) is greater than or equal to 1.2wt. % and less than or equal to 1.8 wt. %. In accordance with thedown-dopant concentration within the depressed cladding region 170, therelative refractive index Δ₃(r) may decrease monotonically withincreasing radial distance from the centerline CL_(i) of the coreportion C_(i) such that the depressed cladding region 170 forms atriangular-shaped trench in the refractive index profile as depicted inFIGS. 6 and 7 . Such embodiments are beneficial in that they may beformed using the single-step process described further herein withrespect to FIGS. 8 and 9 . In embodiments, Δ₃ min is less than or equal−0.2% Δ and greater than or equal to −0.6% Δ.

The radial thickness of a particular glass portion of a core portionC_(i) may be interrelated with a relative refractive index of theparticular glass portion. Specifically, a glass portion ‘i’ with arelative refractive index Δ_(i) %, an inner radius r_(in) and an outerradius r_(out) may have a trench volume V_(i) defined as:V _(i)=2∫_(r) _(in) ^(r) ^(out) Δi % (R)dR  (11)which may be rewritten as:V _(i)=Δ_(i) % (r _(out) ² −r _(in) ²)  (12)Accordingly, the depressed cladding region 170 may have a trench volumeV_(T) of:V _(T)=Δ₃% (r ₃ ² −r ₂ ²)  (13)

In embodiments, the depressed cladding region 170 is constructed to havea down-dopant concentration to achieve a trench volume V_(T) within eachcore portion C_(i) that is greater than or equal to 30% Δ μm² and lessthan 75% Δ μm². Without wishing to be bound by theory, it is believedthat the trench volume V_(T) within the depressed cladding region 170 isdeterminative of the zero dispersion wavelength and the mode fielddiameter of each core portion C_(i). Providing a trench volume V_(T) mayprovide core portions C_(i) having zero dispersion wavelengths greaterthan or equal to 1300 nm and less than or equal 1324 nm, and mode fielddiameters (at 1310 nm) greater than or equal to 8.2 μm and less than orequal to 9.5 μm. Without wishing to be bound by theory, it is believedthat larger trench volumes V_(T) tend to confine the light travellingthrough each core portion C_(i) and make the mode field diameter of eachcore portion C_(i) smaller. In embodiments, if the trench volume isgreater than 75% Δ μm², the mode field diameter tends to be lower than8.2 μm, rendering coupling with a standard single mode fiber moredifficult or have cable cutoff larger than 1260 nm making the fiber notsuitable for operation at 1310 nm wavelength or in the O-band. It isalso believed that the radial starting point of the depressed claddingregion 170 (e.g., r₂ in the depicted embodiments) is also determinativeof the mode field diameter of each core portion C_(i). Relatively larger₂ values reduce the tendency of the depressed cladding region toconfine light propagating through each core portion C_(i). Inembodiments, core portions C_(i) having relatively large r₃ values(e.g., greater than or equal to 15 μm) may comprise trench volumesgreater than or equal to 75% Δ μm². Providing a trench volume V_(T)within such a range may also improve the bend performance of each coreportion C_(i) over multicore fibers not including the depressed claddingregion 170.

FIG. 7 schematically depicts another relative refractive index profilefor the core portions C_(i) of the multicore optical fiber 110 describedherein with respect to FIGS. 1-2 . In embodiments, the core portionrelative refractive index profile depicted in FIG. 7 also extends alongthe line VI shown in FIG. 2 from the centerline CL_(i) of the coreportion C_(i) into the common cladding 19. The core portions C_(i) mayinclude similar structural components as described with respect to FIGS.5 and 6 . As such, in embodiments in accordance with FIG. 7 each of thecore portions C_(i) comprise a core region 150′ and a cladding region155′. The cladding region 155′ comprises an inner cladding region 160′encircling and directly contacting the core region 150′ and a depressedcladding region 170′ encircling and directly contacting the innercladding region 160′. The core region 150′ has a radius r_(1′), and thedepressed cladding region 170′ has a radius r_(3′) that defines an outerradius of the core portion C_(i) such that r_(3′) corresponds to theradius r_(Ci) associated with each core portion C_(i) described hereinwith respect to FIG. 2 . The inner cladding region 160′ extends betweenthe radius r_(1′) of the core region 150′ and an inner radius r_(2′) ofthe depressed cladding region 170′ such that the inner cladding region160 has a thickness T_(2′)=r_(2′)−r_(1′) in the radial direction. Thedepressed cladding region 170′ has a thickness T_(3′)=r_(3′)−r_(2′) inthe radial direction.

Each of the core region 150′, the inner cladding region 160′, and thedepressed cladding region 170′ may have structural and compositionalproperties that are generally similar to those described herein withrespect to the core region 150, the inner cladding region 160, and thedepressed cladding region 170 described herein with respect to FIGS. 5and 6 . As depicted, the core region 150′ depicted in FIG. 7 differsfrom the core region 150 described with respect to FIGS. 5 and 6 in thatthe relative refractive index Δ_(1′)(r) follows a graded index profilehaving a lower alpha value (e.g., less than 2.5) than the alpha value ofthe relative refractive index Δ₁(r) of the core region 150 depicted inFIG. 6 , which has an alpha value of greater than 10. The radius r_(1′)of the core region 150′ is greater than the radius r₁ of the core region150 depicted in FIG. 6 , such that the thickness T_(2′) of the innercladding region 160′ is smaller than the thickness T₂ of the innercladding region 160 depicted in FIG. 6 . The minimum relative refractiveindex Δ_(3 min′) of the depressed cladding region 170′ depicted in FIG.7 is less than the minimum relative refractive index Δ_(3 min) of thedepressed cladding region 170 described herein with respect to FIGS. 5-6, and r_(3′) of the depressed cladding region 170′ is reduced relativeto the r₃ of the depressed cladding region 170 such that the depressedcladding region 170′ defines a slightly smaller trench volume. Therelative refractive index profile depicted in FIG. 6 achieved slightlybetter bend loss performance over core fibers C_(i) constructed inaccordance with the relative refractive index profile depicted in FIG. 7. The performance of the relative refractive profiles depicted in FIGS.6 and 7 , as well as specific values associated therewith, are describedin greater detail in the Examples section contained herein.

In embodiments, the cross talk between each core portion C_(i) and anadjacent core portion C_(i) is less than or equal to −30 dB. The crosstalk depends on the design of the core portions (e.g., the relativerefractive index profiles) and the distance between adjacent coreportions (e.g., the minimum separation distance described herein). Inembodiments, the cross-talk is determined in accordance with equations5-8 herein. In embodiments, the cross-talk between each core portion andan adjacent core portion is less than or equal to −35 dB. Inembodiments, the cross-talk between each core portion and an adjacentcore portion is less than or equal to −40 dB.

In embodiments, each core portion C_(i) of the uncoupled-core multicoreoptical fiber 110 may have an effective area A_(eff) of greater than 62μm² and less than or equal to 72 μm² at a wavelength of 1310 nm. Theeffective area is determined individually for each core portion C_(i) ofthe uncoupled-core multicore optical fiber 110 without consideration ofthe effects of crosstalk between the core portions C_(i) of theuncoupled-core multicore optical fiber 110.

The average attenuation of the uncoupled-core multicore optical fiber110 is determined by measuring the attenuation for each core portionC_(i) of the uncoupled-core multicore optical fiber 110 at a wavelengthof 1310 nm or 1550 nm and then calculating an average attenuation forthe entire uncoupled-core multicore optical fiber 110 based on theindividual attenuation measurements of each core portion C_(i). Inembodiments, the average attenuation at 1310 nm of the uncoupled-coremulticore optical fiber 110 is less than or equal to 0.34 dB/km (e.g.,less than or equal to 0.33 dB/km, less than or equal to 0.32 dB/km). Inembodiments, the average attenuation at 1550 nm of the uncoupled-coremulticore optical fiber 110 is less than 0.19 dB/km (e.g., less than orequal to 0.185 dB/km, less than or equal to 0.18 dB/km). It should beunderstood that the attenuation of the uncoupled-core multicore opticalfiber 110 may be within a range formed from any one of the lower boundsfor attenuation and any one of the upper bounds of attenuation describedherein.

In various embodiments, the cable cutoff of each core portion C_(i) ofthe uncoupled-core multicore optical fiber 110 is greater than or equalto 1100 nm and less than or equal to 1260 nm (e.g., greater than orequal to 1150 nm and less than or equal to 1260 nm). In embodiments, thecable cutoff of each core portion C_(i) of the uncoupled-core multicoreoptical fiber 110 is greater than or equal to 1200 nm and less than orequal to 1260 nm. It should be understood that the cable cutoff of eachcore portion C_(i) of the uncoupled-core multicore optical fiber 110 maybe within a range formed from any one of the lower bounds for cablecutoff and any one of the upper bounds of cable cutoff described herein.

The average 15 mm bend loss of the uncoupled-core multicore opticalfiber is determined by measuring the 15 mm bend loss for each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 at awavelength of 1510 nm and then calculating an average 15 mm bend lossfor the entire uncoupled-core multicore optical fiber based on theindividual 15 mm bend loss measurements of each core portion C_(i). Inembodiments, the average bend loss of the uncoupled-core multicoreoptical fiber 110 measured at a wavelength of 1550 nm using a mandrelwith a 15 mm diameter (“1×15 mm diameter bend loss”) is less than orequal to 0.5 dB/turn or less than or equal to 0.25 dB/turn.

The average 20 mm bend loss of the uncoupled-core multicore opticalfiber is determined by measuring the 20 mm bend loss for each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 at awavelength of 1510 nm and then calculating an average 20 mm bend lossfor the entire uncoupled-core multicore optical fiber 110 based on theindividual 20 mm bend loss measurements of each core portion C_(i). Inembodiments, the average bend loss of the uncoupled-core multicoreoptical fiber 110 at a wavelength of 1550 nm using a mandrel with a 20mm diameter (“1×20 bend loss”) is less than or equal to 0.1 dB/turn orless than or equal to 0.005 dB/turn.

The average 30 mm bend loss of the uncoupled-core multicore opticalfiber 110 is determined by measuring the 30 mm bend loss for each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 at awavelength of 1510 nm and then calculating an average 30 mm bend lossfor the entire uncoupled-core multicore optical fiber 110 based on theindividual 30 mm bend loss measurements of each core portion C_(i). Inembodiments, the average bend loss at 1550 nm of the uncoupled-coremulticore optical fiber 110 at a wavelength of 1550 nm using a mandrelwith a 30 mm diameter (“1×30 bend loss”) is less than or equal to 0.005dB/turn, or less than 0.003 dB/turn or less than or equal to 0.0025dB/turn.

In various embodiments, the zero dispersion wavelength of each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 isgreater than or equal to 1300 nm and less than or equal to 1324 nm. Inembodiments, the zero dispersion wavelength of each core portion C_(i)is greater than or equal to 1308 and less than or equal to 1322. Inembodiments, the zero dispersion wavelength of each core portion C_(i)is greater than or equal to 1310 and less than or equal to 1318. Itshould be understood that the zero dispersion wavelength of each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 may bewithin a range formed from any one of the lower bounds for zerodispersion wavelength and any one of the upper bounds of zero dispersionwavelength described herein.

In various embodiments, dispersion at 1310 nm of each core portion C_(i)of the uncoupled-core multicore optical fiber 110 is greater than orequal to −1.3 ps/nm/km and less than or equal to 1 ps/nm/km. It shouldbe understood that the dispersion at 1310 nm of each core portion C_(i)of the uncoupled-core multicore optical fiber 110 may be within a rangeformed from any one of the lower bounds for dispersion at 1310 nm andany one of the upper bounds of dispersion at 1310 nm described herein.

In various embodiments, the dispersion slope at 1310 nm of each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 isgreater than or equal to 0.085 ps/nm²/km and less than or equal to 0.093ps/nm²/km. It should be understood that the dispersion slope at 1310 nmof each core portion C_(i) of the uncoupled-core multicore optical fiber110 may be within a range formed from any one of the lower bounds fordispersion slope at 1310 nm and any one of the upper bounds ofdispersion slope at 1310 nm described herein.

In various embodiments, dispersion at 1550 nm of each core portion C_(i)of the uncoupled-core multicore optical fiber 110 is greater than orequal to 17 ps/nm/km and less than or equal to 20 ps/nm/km. It should beunderstood that the dispersion at 1550 nm of each core portion C_(i) ofthe uncoupled-core multicore optical fiber 110 may be within a rangeformed from any one of the lower bounds for dispersion at 1550 nm andany one of the upper bounds of dispersion at 1550 nm described herein.

In various embodiments, the dispersion slope at 1550 nm of each coreportion C_(i) of the uncoupled-core multicore optical fiber 110 isgreater than or equal to 0.060 ps/nm²/km and less than or equal to 0.070ps/nm²/km. It should be understood that the dispersion slope at 1550 nmof each core portion C_(i) of the uncoupled-core multicore optical fiber110 may be within a range formed from any one of the lower bounds fordispersion slope at 1550 nm and any one of the upper bounds ofdispersion slope at 1550 nm described herein.

Referring again to FIG. 5 , in embodiments, each core portion C_(i) isfabricated such that the varying relative refractive index Δ₃ of thedepressed cladding region 170 is determined by a down-dopantconcentration D that varies with radial coordinate r, i.e., D=D(r). Inembodiments, the down-dopant is fluorine and D(r) is expressed as aradially-dependent fluorine concentration F(r). As such, F(r) within thedepressed cladding region 170 may vary between a minimum value F_(min)and a maximum value F_(max). In embodiments, F_(min) is at the radialposition r₂ and F_(max) is at the radial position r₃. In embodiments,F_(min) 0 wt %. In embodiments, F_(max) is greater than or equal to 1.2wt. % and less than or equal to 2.0 wt. %. In embodiments, F_(max) isgreater than or equal to 1.2 wt. % and less than or equal to 1.8 wt. %.

The values of the down-dopant concentrations (e.g., F_(max) and F_(min))within the depressed cladding region 170 determine the refractive indexprofile therein, and therefore the trench volume V_(T) of the depressedcladding regions 170 and 170′ in FIGS. 6 and 7 . Without wishing to bebound by theory, it is believed that the trench volume determines a zerodispersion wavelength of each of the core portions C_(i). As describedherein, the down-dopant concentrations may be selected such that thezero dispersion wavelength for each of the core portions C_(i) isgreater than or equal to 1300 nm and less than or equal 1324 nm. Toachieve such a zero dispersion wavelength, the trench volume within thedepressed cladding region 170 of each core portion C_(i) may be 30% Δμm² and less than 75% Δ μm².

The multicore optical fiber 110 of the present disclosure can be madeusing any suitable method for forming a multicore optical fiber. See,for example, U.S. patent application Ser. No. 16/791,708, filed on Feb.14, 2020, the disclosure of which is incorporated herein by reference intheir entirety. Referring now to FIG. 8 by way of example, a flowdiagram of a method 800 of forming a multicore optical fiber isdepicted. The method 800 may be used to form the uncoupled-coremulticore optical fiber 110 (or any of the alternative embodimentsthereof) described herein with respect to FIGS. 1-7 . In a step 802, asoot blank for the common cladding 19 is formed. Formation of the sootblank may involve first forming a soot body via an outside vapordeposition (“OVD”) process, a soot pressing method, a vapor axialdeposition (“VAD”) process, or any other known method. The soot body maybe formed of a glass precursor material. In embodiments, the soot bodyis formed of silica-based material. For example, in an OVD process, aninert rod may be layered with silica-based soot particles that areformed by passing vapors, such as silicon tetrachloride (SiCl₄) vaporsthrough a burner flame such that the vapors react in the flame to formfine silica-based soot particles that are deposited on the inert rod.After soot deposition is complete, the inert rod may be removed and thesoot body may be partially consolidated to reach a bulk densitycompatible for drilling to form the soot blank. The soot blank may thenbe drilled using known techniques to create openings for core caneinsertion.

In a step 804, a core region of a core cane may be formed. Inembodiments, the core region may correspond to the core region 150described herein with respect to FIGS. 5-7 upon completion of the method800 (e.g., after drawing). The core region may be formed via an OVDprocess, a soot pressing method, a VAD process, or any known method. Thecore region may be formed to possess a relative refractive index Δ₁(r)described with respect to FIGS. 5-7 herein after completion of themethod 800. As such, the core region may be formed using an up-dopant.In an example, the core region may be formed using an OVD process whereSiCl₄ vapors are mixed with an up-dopant vapor (e.g.,germanium-containing vapor) and the vapors are passed through a burnerand reacted to form soot particles on an inert rod. In embodiments,after the OVD process is complete, the core region may be partiallyconsolidated by heating the core region to a temperature lower than anormal sintering peak temperature of the material used to form the coreregion for a predetermined time.

In a step 806, an overclad layer is deposited on the on the core region.In embodiments, a soot overclad layer of silica-based soot is formed onthe core region via an OVD or VAD process. In steps 808 and 810, theovercladded core region is positioned within a consolidation furnace andconsolidation of the overcladded core region is initiated. For example,the overcladded core region may be heated to a peak sinteringtemperature to initiate consolidation.

In a step 812, during the consolidation, the overcladded core region isexposed to a down-dopant for a period T after initiation of theconsolidation such that the down-dopant does not reach an inner claddingregion of the overclad upon consolidation. FIG. 9 schematically depictsan example consolidation furnace 914 that may be used to carry out thesteps 808, 810, and 812 described herein. FIG. 9 depicts a soot preform900 resulting from the completion of the consolidation process. The sootpreform 900 includes a core region 902 and an overcladding layer 904surrounding the core region 902. During the step 808, the core region902 (e.g., in an unconsolidated or partially consolidated state) andovercladding layer 904 may be placed in an interior 920 of aconsolidation furnace 914. The consolidation furnace 914 may be heatedto a peak sintering temperature of the overcladding layer 904 toinitiate consolidation.

A gas source 916 is in fluid communication with the interior 920 of theconsolidation furnace 914. The gas source 916 provides a gas 918containing a down-dopant 912 into the interior 920. The down-dopant 912(e.g., fluorine) then diffuses through the overcladding layer 904 duringconsolidation. In embodiments, the rate of diffusion of the down-dopant912 through the overcladding layer is dependent on the compositional andmaterial properties of the overcladding layer 904 (e.g., porosity,density, etc.). As the overcladding layer 904 is consolidated, theporosity of the overcladding layer 904 is diminished such that a rate ofdiffusion of the down-dopant 912 may decrease as the overcladding layer904 consolidates.

As described herein, the core region 902 may contain an up-dopant suchas germanium. The presence of the down-dopant and the up-dopant in thecore region 902 may modify a refractive index profile of the coreportion resulting from performance of the method 800 such that the coreportion does not possess desired properties (e.g., mode field diameter,zero dispersion wavelength, cutoff wavelength, trench volume). As such,the time period T after initiation of the consolidation that thedown-dopant 912 is introduced into the interior 920 may be determinedsuch that the down-dopant 912 does not diffuse through the entirety ofthe overcladding layer 904 prior to the overcladding layer 904 becomingconsolidated. The time period T may be determined based on a rate ofdiffusion of the down-dopant 912 through the overcladding layer 904 andan estimated consolidation time for the soot preform 900. For example,based on a thickness of the overcladding layer 904, the time period Tmay be selected such that only a portion of the overcladding layer 904contains the down-dopant 912 when the soot preform 900 is consolidated.

As depicted in FIG. 9 , an inner cladding region 910 of the overcladdinglayer 904 is substantially free of the down dopant 912 afterconsolidation. Consolidation of the overcladding layer 904 may preventthe down dopant 912 from diffusing into the inner cladding region 910.In embodiments, the inner cladding region 910 corresponds to the innercladding region 160 described herein with respect to FIGS. 5-7 uponcompletion of the method 800. An outer region 906 of the overcladdinglayer 904 possess a variable concentration of the down-dopant 912. Theouter surface 908 of the overcladding layer 904 may have been exposed tothe down-dopant 912 for a greatest time period, and therefore possess ahighest concentration of down-dopant 912. In embodiments, the outerregion 906 corresponds to the depressed cladding region 170 possessing atrench in the relative refractive index profile thereof described hereinwith respect to FIGS. 5-7 upon completion of the method 800.

Referring back to FIG. 8 , in a step 812, after the overcladded coreregion is consolidated into the soot preform 900, the soot preform 900is inserted into holes drilled into the soot blank formed during thestep 802. The steps 804, 806, 808, 810, and 812 may be repeated anynumber of times to insert any number of soot preforms corresponding to adesired number of core portions to be incorporated into theuncoupled-core multimodal optical fiber to form a fiber preform. In astep 816, after each soot preform is inserted into the soot blank, thefiber preform is drawn into a multicore optical fiber.

EXAMPLES

The embodiments described herein will be further clarified by thefollowing examples.

Triangular Trench Examples

Two multicore fiber designs having two different core portion designs(Example A and Example B) were mathematically modeled to determine theoptical properties of the fibers. Each core region of the core portionsof both multicore optical fibers was up-doped with germanium. Inembodiments, each core region is up-doped with germania to comprise amaximum germania concentration of greater than or equal to 6 wt. % andless than or equal to 6.7 wt. %. In embodiments, each of the coreportions also included a depressed cladding region down-doped withfluorine. The depressed cladding regions may comprise a maximum fluorineconcentration that is greater than or equal to 1.6 wt. % and less thanor equal to 1.85 wt. %. Each core portion in both the multicore opticalfibers were modeled with the structure depicted in FIG. 5 . That is,each of the core portions in Examples A and B were modeled to include acore region 150, an inner cladding region 160 encircling and in directcontact with the core region 150, and a depressed cladding region 170encircling and in direct contact with the inner cladding region 160 anddefining a trench in the relative refractive index profiles of the coreportions. Each multicore optical fiber in Examples A and B has an outercommon cladding constructed of undoped silica-based glass having aradius R_(CC)=62.5 μm. Each core portion C_(i) in Example A possessedthe relative refractive index profile depicted in FIG. 6 . Each coreportion C_(i) in Example B possessed the relative refractive indexprofile depicted in FIG. 7 . The structure and optical properties of theoptical fibers of Examples A and B are set forth in Table 1.

Geometric parameters, including, the values of r₁, r₂, and r₃, in μm, ofeach of the core portions as well as R_(CC) of the common cladding inthe described examples, in μm, were determined. Physicalcharacteristics, including, the zero dispersion wavelength in nm of eachcore, the effective area (Δ_(eff)) of the cores in μm², the mode fielddiameter at 1310 nm in microns of each core, the mode field diameter at1510 nm in microns of each core, the cable cutoff in nm of each core,average 1×15 mm diameter bend loss at 1550 nm in dB/turn, average 1×20mm diameter bend loss at 1550 nm in dB/turn, average 1×30 mm diameterbend loss at 1550 nm in dB/turn, the dispersion at 1310 nm in ps/nm/kmof each core, dispersion slope at 1310 nm in ps/nm²/km of each core, thedispersion at 1550 nm in ps/nm/km of each core, and the dispersion slopeat 1550 nm in ps/nm²/km of each core, were also determined.

TABLE 1 Examples A and B Parameter Example A Example B Maximum CoreIndex, Δ_(1max) (%) 0.336 0.37 Core Region Radius, r₁, microns 4.2 5.3Core α 12 2.2 Inner Cladding Index, Δ₂ (%) 0 0 Inner Cladding RegionRadius, r₂ 7.16 7.45 (microns) Depressed Cladding Region ShapeTriangular Triangular Depressed Cladding Region Minimum −0.5 −0.55Index, Δ_(3min) (%) Depressed Cladding Region Outer 15.9 14.9 Radius, r₃(micron) Volume of First Depressed Cladding −56.9 −50.94 Region, V_(T),% Δ micron² Common Cladding Index, Δ_(CC) (%) 0 0 Common CladdingRadius, R_(CC), microns 62.5 62.5 Mode Field Diameter (micron) at 13109.1 9.1 nm Effective Area at 1310 nm (micron²) 64.5 62.4 Zero DispersionWavelength (nm) 1314 1319 Dispersion at 1310 nm (ps/nm/km) −0.36 −0.837Dispersion Slope at 1310 nm (ps/nm2/km) 0.090 0.093 Mode Field Diameter(micron) at 1550 10.21 10.22 nm Effective Area at 1550 nm (micron²)79.78 78.69 Dispersion at 1550 nm (ps/nm/km) 18.32 18.27 DispersionSlope at 1550 nm (ps/nm2/km) 0.064 0.065 Cable Cutoff (nm) 1226 1204Bend Loss for 15 mm mandrel diameter at 0.093 0.123 1550 nm (dB/turn)Bend Loss for 20 mm mandrel diameter at 0.023 0.113 1550 nm (dB/turn)Bend Loss for 30 mm mandrel diameter at 0.0025 0.004 1550 nm (dB/turn)

As shown in Examples A and B, the optical fibers described herein arecapable of achieving an effective area A_(eff) at 1310 nm for each coreportion of greater than or equal to 60 μm² and less than or equal to 72μm². In embodiments, the optical fibers described herein are capable ofachieving an effective area Δ_(eff) at 1310 nm for each core portion ofgreater than or equal to 63 μm² and less than or equal to 70 μm². Theoptical fibers described herein also demonstrate a mode field diameterat 1310 nm of greater than or equal to 9 μm and less than or equal to9.5 μm to facilitate coupling with a standard single mode fibers. Inembodiments, the optical fibers described herein have a mode fielddiameter of greater than or equal to 9.1 μm and less than or equal to9.2 μm to facilitate coupling with a standard single mode fiber. Theoptical fibers described herein also demonstrate a cable cutoff of lessthan or equal to 1260 nm (e.g., less than or equal to 1230 nm),demonstrating capacity of the core portions herein for single modetransmission.

As shown in Examples A and B, each core portion of the optical fibersdescribed herein have a zero dispersion wavelength greater than or equalto 1300 nm and less than or equal to 1324 nm to facilitate long-termtransmission of optical signals within that wavelength range. While notshown in Examples A and B in embodiments, the core-portions of each ofthe optical fibers described herein have a cross talk of less than orequal to −30 dB (e.g., less than or equal to −40 dB, or even less thanor equal to −50 dB) with adjacent core portions. To achieve cross talkvalues in such ranges, the core portions of the optical fibers describedherein may be separated from one another by at least a minimumseparation distance that is greater than or equal to 30 μm (e.g.,greater than or equal to 35 μm, greater than or equal to 40 μm). Inembodiments, to prevent tunneling loss within the optical fibersdescribed herein, core portions of the optical fibers herein may beseparated from an outer edge (e.g., an outer edge of a common cladding)by at least a minimum core edge to fiber edge distance that is greaterthan or equal to 18 μm (e.g., greater than or equal to 20 μm, greaterthan or equal to 25 μm).

Rectangular Trench Examples

In additional examples, another two multicore fiber designs having twodifferent core portion designs (Example C and Example D) weremathematically modeled to determine the optical properties of thefibers. Each core region of the core portions of both multicore opticalfibers was up-doped with germanium. In embodiments, each core region isup-doped with germania to comprise a maximum germania concentration ofgreater than or equal to 6 wt. % and less than or equal to 6.7 wt. %. Inembodiments, each of the core portions also included a depressedcladding region down-doped with fluorine. The depressed cladding regionsmay comprise a maximum fluorine concentration that is greater than orequal to 1.5 wt. % and less than or equal to 1.7 wt. %. Each coreportion in both the multicore optical fibers were modeled with thestructure depicted in FIG. 5 . That is, each of the core portions inExamples C and D were modeled to include a core region 150, an innercladding region 160 encircling and in direct contact with the coreregion 150, and a depressed cladding region 170 encircling and in directcontact with the inner cladding region 160 and defining a trench in therelative refractive index profiles of the core portions. Each multicoreoptical fiber in Examples C and D has an outer common claddingconstructed of undoped silica-based glass having a radius R_(CC)=62.5μm. Examples C and D differ from the Examples A and B described hereinin that the Examples C and D include depressed cladding regions 170 witha rectangular trench profile. That is, the relative refractive index Δ₃remains substantially constant within the depressed cladding regions 170of examples C and D at the minimum relative refractive index Δ_(3 min).In embodiments, the depressed cladding regions 170 define a trenchhaving a trench volume V_(T) greater than or equal to 40% Δ μm² and lessthan or equal to 70% Δ μm².

Geometric parameters, including, the values of r₁, r₂, and r₃, in μm, ofeach of the core portions C_(i), as well as R_(CC) of the commoncladding in the described examples, in μm, were determined. Physicalcharacteristics, including, the zero dispersion wavelength in nm of eachcore, the effective area (Δ_(eff)) of the cores in μm², the mode fielddiameter at 1310 nm in microns of each core, the mode field diameter at1510 nm in microns of each core, the cable cutoff in nm of each core,average 1×15 mm diameter bend loss at 1550 nm in dB/turn, average 1×20mm diameter bend loss at 1550 nm in dB/turn, average 1×30 mm diameterbend loss at 1550 nm in dB/turn, the dispersion at 1310 nm in ps/nm/kmof each core, dispersion slope at 1310 nm in ps/nm²/km of each core, thedispersion at 1550 nm in ps/nm/km of each core, and the dispersion slopeat 1550 nm in ps/nm²/km of each core, were also determined.

TABLE 2 Examples C and D Parameter Example C Example D Maximum CoreIndex, Δ_(1max) (%) 0.336 0.37 Core Region Radius, r₁, microns 4.2 5.3Core α 12 2.2 Inner Cladding Region Index, Δ₂ (%) 0 0 Inner CladdingRegion Radius, r₂ 9.14 10.56 (microns) Depressed Cladding Region ShapeRectangular Rectangular Depressed Cladding Region Minimum −0.45 −0.5Index, Δ_(3,min) (%) Depressed Cladding Region Outer 14.2 14.6 Radius,r₃ (micron) Volume of Depressed Cladding Region, −53.14 −50.82 V_(T) % Δmicron² Common Cladding Index, Δ_(CC) (%) 0 0 Mode Field Diameter(micron) at 1310 9.07 9.26 nm Effective Area at 1310 nm (micron²) 64.565.29 Zero Dispersion Wavelength (nm) 1310 1318 Dispersion at 1310 nm(ps/nm/km) 0 −0.736 Dispersion Slope at 1310 nm 0.092 0.092 (ps/nm2/km)Mode Field Diameter (micron) at 1550 10.14 10.45 nm Effective Area at1550 nm (micron²) 79.16 82.43 Dispersion at 1550 nm (ps/nm/km) 19.0318.35 Dispersion Slope at 1550 nm 0.066 0.066 (ps/nm2/km) Cable Cutoff(nm) 1215 1200 Bend Loss for 15 mm mandrel diameter 0.118 0.138 at 1550nm (dB/turn) Bend Loss for 20 mm mandrel diameter 0.04 0.082 at 1550 nm(dB/turn) Bend Loss for 30 mm mandrel diameter 0.0063 0.012 at 1550 nm(dB/turn)

As shown in Examples C and D, the optical fibers described herein arecapable of achieving an effective area Δ_(eff) at 1310 nm for each coreportion of greater than or equal to 60 μm² and less than or equal to 72μm². In embodiments, the optical fibers described herein are capable ofachieving an effective area Δ_(eff) at 1310 nm for each core portion ofgreater than or equal to 63 μm² and less than or equal to 70 μm². Theoptical fibers described herein also demonstrate a mode field diameterat 1310 nm of greater than or equal to 9 μm and less than or equal to9.5 μm to facilitate coupling with a standard single mode fibers. Theoptical fibers described herein also demonstrate a cable cutoff of lessthan or equal to 1260 nm (e.g., less than or equal to 1230 nm),demonstrating capacity of the core portions herein for single modetransmission.

As shown in Examples C and D, each core portion of the optical fibersdescribed herein have a zero dispersion wavelength greater than or equalto 1300 nm and less than or equal to 1324 nm to facilitate long-termtransmission of optical signals within that wavelength range. While notshown in Examples C and D in embodiments, the core-portions of each ofthe optical fibers described herein have a cross talk of less than orequal to −30 dB (e.g., less than or equal to −40 dB, or even less thanor equal to −50 dB) with adjacent core portions.

As is apparent from the foregoing description, uncoupled-core multicoreoptical fibers comprising a plurality of core portions with depressedcladding regions surrounding core regions provide relatively lowcross-talk among the core portions while achieving relatively high fiberdensity. Additionally, such depressed cladding regions providerelatively low bend loss for the multicore optical fibers. Depressedcladding regions with relative refractive indexes that decreasemonotonically with increasing radius may beneficially be produced by amethod where the depressed cladding region is consolidated in a singlestep with a core region having a refractive index during doping.Embodiments of the present disclosure facilitate incorporating of aplurality of core portions (e.g., greater than or equal to 3 coreportions and less than or equal to eight core portions) into a standard125 μm optical fiber while still providing relatively low cross-talk(e.g., less than −30 dB) while maintaining a mode field diameter of eachcore portion to greater than or equal to 8.2 μm to facilitate couplingwith standard single mode fibers.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A multicore optical fiber comprising: a commoncladding; and a plurality of core portions disposed in the commoncladding, each of the plurality of core portions comprising: a centralaxis; a core region extending from the central axis to a radius r₁, thecore region comprising a relative refractive index Δ₁ relative to puresilica; an inner cladding region encircling and directly contacting thecore region and extending from the radius r₁ to a radius r₂, the innercladding region comprising a relative refractive index Δ₂ relative topure silica; and a depressed cladding region encircling and directlycontacting the inner cladding region and extending from the radius r₂ toa radius r₃, the depressed cladding region comprising a relativerefractive index Δ₃ relative to pure silica and a minimum relativerefractive index Δ_(3 min) relative to pure silica, wherein:Δ₁>Δ₂>Δ_(3 min); the relative refractive index Δ₃ of the entiredepressed cladding region in each of the plurality of core portions isless than or equal to Δ₂ such that the depressed cladding region forms adepressed-index trench in a relative refractive index profile of eachcore portion; the depressed-index trench in the relative refractiveindex profile of each core portion has a trench volume of greater thanor equal to 30% Δ μm² and less than or equal to 75% Δ μm²; the modefield diameter of each core portion is greater than or equal to 8.2 μmand less than or equal to 9.5 μm at a 1310 nm wavelength; and the zerodispersion wavelength of each core portion is greater than or equal to1300 nm and less than or equal to 1324 nm.
 2. The multicore opticalfiber of claim 1, wherein the common cladding comprises an outer radiusR_(CC) that is greater than or equal to 120 μm and less than or equal to200 μm.
 3. The multicore optical fiber of claim 2, wherein the pluralityof core portions comprises greater than or equal to 3 core portions andless than or equal to 8 core portions.
 4. The multicore optical fiber ofclaim 3, wherein the outer radius R_(CC) is equal to 125 μm.
 5. Themulticore optical fiber of claim 3, wherein the plurality of coreportions are arranged in a 2×2 arrangement within the common claddingsuch that each central axis of a core portion of the plurality of coreportions is separated from central axes of two adjacent core portions bya minimum core-to-core separation distance greater than or equal to 35μm.
 6. The multicore optical fiber of claim 1, wherein a cable cutoffwavelength of each of the plurality of core portions is less than orequal to 1260 nm.
 7. The multicore optical fiber of claim 1, wherein therelative refractive index Δ₃ of the depressed cladding region of eachcore portion monotonically decreases from Δ₂ at the radius r₂ toΔ_(3 min) at r₃.
 8. The multicore optical fiber of claim 7, wherein therelative refractive index Δ₃ of the depressed cladding region of eachcore portion continuously decreases from Δ₂ at the radius r₂ toΔ_(3 min) at r₃ such that the trench has a substantiallytriangular-shape.
 9. The multicore optical fiber of claim 7, wherein therelative refractive index Δ_(3 min) of the depressed cladding region ofeach core portion is less than or equal −0.2% Δ and greater than orequal to −0.6% Δ.
 10. The multicore optical fiber of claim 7, whereinthe depressed cladding region of each core portion comprises adown-dopant having a concentration that varies with radial distance fromthe central axis such that the depressed cladding region comprises amaximum down-dopant concentration at r₃ and a minimum down-dopantconcentration at r₂.
 11. The multicore optical fiber of claim 10,wherein the down-dopant is fluorine and the maximum down-dopantconcentration is greater than or equal to 1.2 wt % and less than orequal to 2.0 wt %.
 12. The multicore optical fiber of claim 10, whereinthe inner cladding region of each of the core portions is substantiallyfree of the down-dopant.
 13. The multicore optical fiber of claim 1,wherein the central axes of the plurality of core portions are separatedfrom one another by a minimum separation distance that is greater thanor equal to 35 microns.
 14. The multicore optical fiber of claim 13,wherein a cross-talk between each of the plurality of core portions anda nearest one of the plurality of core portions is less than or equal to−30 dB.
 15. A multicore optical fiber comprising: a common cladding; anda plurality of core portions disposed in the common cladding, each ofthe plurality of core portions comprising: a central axis; a core regionextending from the central axis to a radius r₁, the core regioncomprising a relative refractive index Δ₁ relative to pure silica; aninner cladding region encircling and directly contacting the core regionand extending from the radius r₁ to a radius r₂, the inner claddingregion comprising a relative refractive index Δ₂ relative to puresilica; and a depressed cladding region extending encircling anddirectly contacting the inner cladding region and from the radius r₂ toa radius r₃, the depressed cladding region comprising a relativerefractive index Δ₃ relative to pure silica and a minimum relativerefractive index Δ_(3 min) relative to pure silica, wherein:Δ₁>Δ₂>Δ_(3 min); Δ₂≥Δ₃ such that the depressed cladding region forms adepressed-index trench in a relative refractive index profile of eachcore portion between r₂ and r₃; and Δ₃ monotonically decreases to theminimum relative refractive index Δ_(3 min) with increasing radialdistance from the central axis of each core portion.
 16. The multicoreoptical fiber of claim 15, wherein the mode field diameter of each coreportion is greater than or equal to 8.2 μm and less than or equal to9.5.
 17. The multicore optical fiber of claim 15, wherein the zerodispersion wavelength of each core portion is greater than or equal to1300 nm and less than or equal to 1324 nm.
 18. The multicore opticalfiber of claim 15, wherein a cable cutoff wavelength of each of theplurality of core portions is less than or equal to 1260 nm.
 19. Themulticore optical fiber of claim 15, wherein each core region comprisesa maximum relative refractive index Δ_(1 max) relative to pure silica,wherein Δ_(1 max) in each of the core portions is greater than or equalto 0.28% Δ and less than or equal to 0.45% Δ.
 20. The multicore opticalfiber of claim 15, wherein the common cladding comprises an outer radiusR_(CC) that is greater than or equal to 120 μm and less than or equal to200 μm.
 21. The multicore optical fiber of claim 20, wherein theplurality of core portions comprises greater than or equal to 3 coreportions and less than or equal to 8 core portions.
 22. The multicoreoptical fiber of claim 15, wherein the relative refractive index Δ₃ ofthe depressed cladding region of each core portion continuouslydecreases from Δ₂ at the radius r₂ to Δ_(3 min) at r₃ such that thetrench has a substantially triangular-shape.